Drop Python 2.7 and 3.4 support

* Update setup.py requires and classifiers
* Drop Python 2.7 and 3.4 from CI
* Update docs dropping 2.7 and 3.4 support
* Fix mock imports and remove tests related to pypi's mock module
* Add py27 and 34 support docs to the sidebar
* Remove usage of six from tmpdir
* Remove six.PY* code blocks
* Remove sys.version_info related code
* Cleanup compat
* Remove obsolete safe_str
* Remove obsolete __unicode__ methods
* Remove compat.PY35 and compat.PY36: not really needed anymore
* Remove unused UNICODE_TYPES
* Remove Jython specific code
* Remove some Python 2 references from docs

Related to #5275

src/_pytest/tmpdir.py

@@ -11,7 +11,6 @@ import warnings
 
 import attr
 import py
-import six
 
 import pytest
 from .pathlib import ensure_reset_dir
@@ -32,9 +31,7 @@ class TempPathFactory(object):
         # using os.path.abspath() to get absolute path instead of resolve() as it
         # does not work the same in all platforms (see #4427)
         # Path.absolute() exists, but it is not public (see https://bugs.python.org/issue25012)
-        converter=attr.converters.optional(
-            lambda p: Path(os.path.abspath(six.text_type(p)))
-        )
+        converter=attr.converters.optional(lambda p: Path(os.path.abspath(str(p))))
     )
     _trace = attr.ib()
     _basetemp = attr.ib(default=None)